1. Field of the Invention.
The present invention relates to apparatus for suppressing optical frequencies which are in excess of the Nyquist frequency for use in optical systems such as autofocus cameras.
2. Description of the Prior Art.
In autofocus systems, such as that described in Norman L. Stauffer U.S. Pat. No. 4,185,191, assigned to the assignee of the present invention, a problem is encountered when the scene being viewed contains high spatial frequencies. More particularly, when frequencies in excess of the Nyquist frequency are imaged on the lenslets of the autofocus system, the number of data points becomes less than 2 for each cycle with the result that the information obtained from the samples becomes inaccurate and the frequency produced by the samples becomes erroneous. The phase from the signal from the sample reverses when the spatial frequency is between the Nyquist frequency and twice the Nyquist frequency. Accordingly, the information in such high spatial frequencies is not useful to the autofocus system and can produce undesirable errors. There are currently no optical filters available that will suppress frequencies in excess of the Nyquist frequency and accordingly, other approaches to suppressing such frequencies have been proposed. In the Utagawa U.S. Pat. No. 4,460,260, a solution is proposed wherein multiple rows of lenslets are arranged in a mutually parallel fashion and the outputs of the detectors behind each of the lenslets is summed from one row to the next to produce an output. The lenslets are staggered or arranged in such a fashion that high frequency information is presented to the lenslets over different portions of the surface thereof from one row to another. Thus, the effect of the high frequency scene on the lenslets is different from one parallel row to the next and the summed output of the detectors does not respond to these higher frequencies to the same extent as the detectors respond to lower frequencies. Accordingly, the offsetting images operate to suppress the higher frequencies in a desirable manner.
The difficulty encountered with the solution presented by the Utagawa patent is the requirement for having pluralities of rows of lenslets spaced accurately in parallel fashion and connecting the outputs of the detectors together. The lenslet detector combinations are difficult to manufacture and the requirement of three or more parallel rows of such combinations is extremely difficult to achieve.